I am still reading more on your post and others in this thread as well as others. Trying to reach a conclusion on thin vs thick still.
It really doesn't have to be a "vs." thing. Despite what we've seen in manuals lately (and even GM manuals from over twenty years ago), an engine can be served by a few viscosity options under several circumstances, and even more options when the circumstances are out at the limits.
GM years ago warned against 10w-40 not because 40 grades were unusable, but because they had issues with the 10w-40 options of the time. Many current vehicles call for a 0w-20 because there are fuel economy benefits, not because they are unable to muster the cranking power to get a 5w-30 flowing at 70 F. The Germans often call for a proprietary, higher HTHS specification because they use very extended OCIs and may face more stressed driving conditions (note that most approved lubes are also winter suitable synthetics, too) because their earlier call for API/ILSAC lubes had some pretty enormous OCIs.
Even leaving CAFE aside and 10w-40 issues of decades ago, there is something to be said for simplicity. There is nothing wrong with trying to eke out a bit of fuel economy (assuming it doesn't cause other major issues). There's nothing wrong with at least attempting to call for a lubricant that will work under almost any condition the motorist will find. Yes, BITOGers generally know that running a 20w-50 in a Saskatchewan winter is foolhardy and a the cold cranking properties of a 0w-XX won't be tested in Arizona. You let the general public run 20w-50 in July, though, and they put 1000 miles on the car, they won't want to change in December, particularly when their manual or OLM indicates they have another 9000 miles to go. Similarly, you let someone get away with the cheapest option for the longest interval, some people will try it. Well, they will anyhow, but if you specify something like a 0w-20, it'll probably be a synthetic, and probably be suitable for a longer OCI, and certainly be okay in the winter.
In that vein, look at what I eventually did with my Audi Turbo 200's service. It had a couple of the issues I addressed above. It allowed everything from a 5w-20 to a 20w-50 to monogrades, depending upon ambient, all with a 12500 km severe service interval on basic API SJ. 15w-40 was the preferred grade over the widest temperatures. Should I have been running 5w-20 in the winter and then running to 15w-40 in spring? Well, I did a bit of 5w-30 in the winter and then ran to 15w-40 in the summer, and then ran into fall again fairly quickly, and that got old pretty quick with only putting 2500 km on the oil. So, I grabbed a 0w-40 A3/B4 or 5w-40 E7, E9 synthetic (not even an option in the manual back then, as I recall) and kept it all year.
The point I'm making isn't that it's necessarily about thick or thin. Follow the manual. If you can't or won't, choose some reasonable range of HTHS, select an appropriate number before the "w" for your climate, an appropriate API or ACEA or builder spec, and run the product year round. If that happens to be a monograde, and that works satisfactorily for you, fine. If it's an A3/B4, fine. If it's a 0w-20 ILSAC oil, that's fine, too.
My Audi would work just fine running a 5w-20 (or a 0w-20) in the winter and a 20w-50 in the summer. I, on the other hand, would stroke out dumping oil at one fifth of the severe service interval just to make sure I can start the car when the ugly weather hits.
There's no real conclusion to reach. You're not going to make your car a Guinness entry for longevity by running 20w-50 or SAE 30 all year, nor will you make yourself immune to winter problems by running a 0w-XX, nor will you slice your gas bills in half by using a 20.
